In an earlier contribution to this discussion,
Get yourself a Haynes manual for your car. If it's like the one for my 1999
V70, it will have a fairly comprehensive set of wiring diagrams.
Assuming the 850 has a bulb failure unit like the later V70, you'll have to
tap into the *input* side of this for those items which it monitors -
notably tail and stop lamps. The rest can be connected very close to the
lighting clusters - where it's fairly easy to work out what's what.